1. Field of the Invention
This invention is in the field of combustion processes for piston internal combustion engines, and particularly for such engines using compression ignition to initiate the burning of the principal air fuel mixture.
2. Description of the Prior Art
In addition to conventional diesel engines, some natural gas engines also use compression ignition to initiate the burning of the air fuel mixture. In these natural gas engines, a very fuel lean mixture of natural gas in air is the principal air fuel mixture for the engine. This lean principal mixture is difficult to ignite with a conventional electric spark and, when thusly spark ignited, burns slowly, thus reducing the engine efficiency. Liquid diesel fuel can be injected into the highly compressed lean principal mixture and the compression ignition of this injected diesel fuel ignites the lean principal mixture at many different places. By thusly starting burning at many places, the burning of the lean principal mixture can be completed in a short time interval and hence with good engine efficiency. In this way lean principal natural gas in air mixtures can be used efficiently and are known to have lower emissions of some undesirable exhaust components than stoichiometric mixtures.
All prior art compression ignition engines inject a liquid fuel into the engine combustion chamber. After injection and atomization a portion of the liquid fuel evaporates and diffuses into the surrounding air or natural gas in air mixture. In consequence almost the entire range of air to fuel ratios is created in separate regions surrounding the injected and atomized liquid fuel. Some of these separate regions will have an air to fuel ratio at which the compression ignition delay time is a minimum and these will be the regions first to ignite. Burning is thus initiated in these regions whose air to fuel ratio is that for minimum compression ignition delay time. Burning progresses from these first to ignite regions into those other regions containing both evaporated fuel and air as needed to sustain burning. Those liquid fuel portions, unevaporated at the time of initiation of burning, will thus become surrounded with very high temperature burned gases largely devoid of oxygen. These initially unevaporated liquid fuel portions will subsequently evaporate quickly, and be heated to a high temperature, but will have difficulty finding the oxygen needed for burnup. In consequence these fuel portions are partially converted to soot which is very slow to burn. While some of this soot will subsequently burn slowly within the engine, other soot portions will survive unburned into the engine exhaust as undesirable smoke emissions. Such exhaust smoke is undesirable, not only as being unsightly, but also as causing reduced engine efficiency, since not all fuel is burned. Additionally these soot particles aggravate the engine exhaust odor problem by absorbing, and hence concentrating, odor creating exhaust components, such as formaldehyde.
It would be very desirable to devise a compression ignition process, for use in internal combustion engines, which produced little or no soot.
3. Definitions
The term compression ignition combustion process is used herein and in the claims to mean a process wherein a mixture of air and fuel is sufficiently compressed that it self ignites, in one or more regions, without an external ignition source, and combustion can be initiated in this way.
The term piston internal combustion engine is used herein and in the claims to mean an internal combustion engine of the piston and cylinder type, or equivalent, such as the Wankel engine type, and comprising a variable volume chamber enclosed by the piston and cylinder. Gases contained within this variable volume chamber are alternately compressed and expanded as the chamber volume is decreased and increased. A piston internal combustion engine can be of the two strokes per cycle type or of the four strokes per cycle type. Additional details of piston internal combustion engines are described in the background portion of my earlier filed application entitled, "Displacer Jet Igniter", Ser. No. 08/368,093, FILED Jan. 3, 1995, and this material is incorporated herein by reference thereto.
The term, principal air fuel mixture, fuel leaner than the minimum compression ignition delay time fuel air ratio, herein and in the claims, encompasses such mixtures containing fuel, and also air to which no fuel has been added.